In a conventional vehicle air handling system, a heater core is mounted in an air duct downstream of an evaporator core. Engine coolant is circulated through the heater core to heat the air. For air conditioning, refrigerant is circulated through the evaporator core. Control of the air temperature is obtained by controlling the flow of air from the evaporator core to the heater core. For maximum cooling, all of the air flow from the evaporator core bypasses the heater core and travels directly to the passenger compartment without contacting the heater core. Alternatively, for maximum heating, all of the air flow from an inactive evaporator core passes through the heater core, where the air is heated, before traveling to the passenger compartment. Intermediate these two extremes, bypass flow and flow through the heater core mix to provide an air flow to the passenger compartment of an intermediate temperature.
The amount of air which bypasses the heater core and the amount of air which flows through the heater core are typically controlled by a movable member which moves to block flow from the evaporator core to the heater core or to block bypass flow from the evaporator core directly to the passenger compartment. The movable member may, in certain positions, allow both bypass flow and flow through the heater core. Normally, the movable member moves in a frame that has an opening through which the air flows. The movable member is in the nature of a door which blocks air flow through a portion of the opening and can totally block air flow to the heater core or totally block bypass air flow. It is desirable that movement of the door be accomplished reliably and with minimum effort.